1. Field
The present disclosure relates to a tire for an aeroplane and in particular to the crown of a tire for an aeroplane.
2. Description of Related Art
A tire comprises a crown comprising a tread intended to come into contact with the ground via a tread surface, two beads intended to come into contact with a rim and two sidewalls connecting the crown to the beads. A radial tire, as is generally used for an aeroplane, more particularly comprises a radial carcass reinforcement and a crown reinforcement, both as described, for example, in document EP 1 381 525.
As a tire has a geometry exhibiting symmetry of revolution about an axis of rotation, the geometry of the tire is generally described in a meridian plane containing the axis of rotation of the tire. For a given meridian plane, the radial, axial and circumferential directions respectively denote the directions perpendicular to the axis of rotation of the tire, parallel to the axis of rotation of the tire, and perpendicular to the meridian plane.
In what follows, the expressions “radially on the inside of” and “radially interior” respectively signify “closer to the axis of rotation of the tire, in the radial direction, than” and “closest to the axis of rotation of the tire in the radial direction”. The expressions “radially on the outside of” and “radially exterior” respectively signify “further away from the axis of rotation of the tire, in the radial direction, than” and “furthest from the axis of rotation of the tire, in the radial direction”. The expressions “axially on the inside of” and “axially interior” respectively signify “closer to the equatorial plane, in the axial direction, than” and “closest to the equatorial plane, in the axial direction”, the equatorial plane being the plane perpendicular to the axis of rotation of the tire and passing through the middle of the tread surface of the tire. Similarly, the expressions “axially on the outside of” and “axially exterior” respectively signify “further from the equatorial plane, in the axial direction, than” and “furthest from the equatorial plane, in the axial direction”. A “radial distance” is a distance with respect to the axis of rotation of the tire, and an “axial distance” is a distance with respect to the equatorial plane of the tire. A “radial thickness” is measured in the radial direction, and an “axial width” is measured in the axial direction.
The radial carcass reinforcement is the tire reinforcing structure that connects the two beads of the tire. The radial carcass reinforcement of a tire for an aeroplane generally comprises at least one carcass reinforcing layer, referred to as the carcass layer. Each carcass layer is made up of reinforcing elements coated in a polymer material, parallel to one another and making with the circumferential direction an angle of between 80° and 100°. Each carcass layer is individual, i.e. contains just one reinforcing element within its thickness.
The crown reinforcement is the tire reinforcing structure radially on the inside of the tread and usually radially on the outside of the radial carcass reinforcement. The crown reinforcement for a tire of an aeroplane generally comprises at least one crown reinforcing layer, referred to as the crown layer. Each crown layer is made up of reinforcing elements coated in a polymer material, parallel to one another and making with the circumferential direction an angle of between +20° and −20°. Each crown layer is individual, i.e. contains just one reinforcing element in its thickness.
Of the crown layers, the distinction is made between the working layers that make up the working reinforcement, usually made up of textile reinforcing elements, and the protective layers that make up the protective reinforcement, made up of metal or textile reinforcing elements and arranged radially on the outside of the working reinforcement. The working layers govern the mechanical behaviour of the crown. The protective layers essentially protect the working layers from attack likely to spread through the tread radially towards the inside of the tire. A crown layer, particularly a working layer, is often an axially wide layer, i.e. one that has an axial width, for example, at least equal to two-thirds of the maximum axial width of the tire. The maximum axial width of the tire is measured at the sidewalls, with the tire mounted on its rim and lightly inflated, i.e. inflated to a pressure equal to 10% of the nominal pressure as recommended, for example, by the Tire and Rim Association or TRA.
The tire may also comprise a hooping reinforcement, radially on the inside or radially on the outside of the crown reinforcement, or even inserted between two crown layers. The hooping reinforcement of an aeroplane tire generally comprises at least one layer of hooping reinforcement, referred to as a hooping layer. Each hooping layer is made up of reinforcing elements coated in a polymer material, parallel to one another and making with the circumferential direction an angle of between +10° and −10°. A hooping layer is usually an axially narrow working layer, i.e. one that has an axial width which is appreciably smaller than the axial width of a crown layer and, for example, at most equal to half the maximum axial width of the tire.
The reinforcing elements of the carcass, working and hooping layers for aeroplane tires are usually cords made up of spun textile filaments, preferably made of aliphatic polyamides or aromatic polyamides. The reinforcing elements of the protective layers may be either cords consisting of metal threads or cords made up of spun textile filaments.
As far as the textile reinforcing elements are concerned, the mechanical tensile properties (modulus, elongation and braking force) of the textile reinforcing elements are measured after prior conditioning. What is meant by “prior conditioning” is that the textile reinforcing elements are stored for at least 24 hours prior to measurement, in a standard atmosphere in accordance with European standard DIN EN 20139 (temperature of 20±2° C.; relative humidity of 65±2%). The measurements are carried out in a known way using a ZWICK GmbH & Co (Germany) tensile testing machine of type 1435 or type 1445. The textile reinforcing elements are tensile tested over an initial length of 400 mm at a nominal rate of 200 mm/min. All the results are averaged over ten measurements.
Aeroplane tires often exhibit non-uniform tread wear, referred to as uneven wear, as a result of the demands made of them during the various phases of the life of the tire: take-off, taxiing and landing. It has been more particularly demonstrated that there is differential tread wear between a middle part and the two lateral parts of the tread, axially on the outside of the middle part. It is usually desirable for the wear in the middle part to be greatest and to govern the life of the tire. In some instances, the aforementioned differential wear exacerbates the wearing of the lateral parts of the tread, which then becomes predominant over the wear of the middle part, leading to premature withdrawal from service which is economically detrimental.
Those skilled in the art know that tire tread wear is dependent on various factors associated with the use and design of the tire. Wear is particularly dependent on the geometric shape of the contact patch via which the tire tread makes contact with the ground and on the distribution of mechanical stresses within this contact patch. These two parameters are dependent on the inflated meridian profile of the tread surface. The inflated meridian profile of the tread surface is the cross section through the tread surface, on a meridian plane, for a tire inflated to its nominal pressure and unladen.
In order to increase the life of the tire with respect to the differential wear of the middle part of the tread, a person skilled in the art has sought to optimize the geometric shape of the inflated meridian profile of the tread surface.
Document EP 1 163 120 discloses a crown reinforcement for an aeroplane tire in which attempts have been made to limit the radial deformations when the tire is being inflated to its nominal pressure, making it possible to limit the radial deformations of the inflated meridian profile of the tread surface. The radial deformations of the crown reinforcement when the tire is being inflated to its nominal pressure is successfully limited by increasing the circumferential tensile stiffnesses of the crown layers, this being obtained by replacing the crown layer reinforcing elements, which are usually made of aliphatic polyamides, with reinforcing elements made of aromatic polyamides. Because the moduli of elasticity of reinforcing elements made of aromatic polyamides are higher than those of reinforcing elements made of aliphatic polyamides, the elongations of the former, for a given tensile loading, are smaller than those of the latter.
The aforementioned document EP 1 381 525 proposes one approach which is to alter the geometric shape of the inflated meridian profile of the tread surface by altering the tensile stiffnesses of the crown and/or carcass layers. That document proposes the use of hybrid reinforcing elements, namely reinforcing elements made both of aliphatic polyamides and of aromatic polyamides, in place of the usual reinforcing elements made of aliphatic polyamides. These hybrid reinforcing elements have moduli of elasticity that are higher than those of the reinforcing elements made of aliphatic polyamides, and therefore have lower elongations, for a given tensile loading. The hybrid reinforcing elements are used in the crown layers to increase the circumferential tensile stiffnesses and/or in the carcass layers to increase the tensile stiffnesses in the meridian plane.
Document EP 1 477 333 proposes another approach which is to alter the geometric shape of the inflated meridian profile of the tread surface by axially altering the overall circumferential tensile stiffness of the crown reinforcement in such a way that the ratio between the overall circumferential tensile stiffnesses of the axially outermost parts of the crown reinforcement and of the middle part of the crown reinforcement lies within a defined range. The overall circumferential tensile stiffness of the crown reinforcement is a result of the combination of the circumferential tensile stiffnesses of the crown layers. The overall circumferential tensile stiffness of the crown reinforcement varies in the axial direction according to changes in the number of superposed crown layers. The proposed solution is based on an axial distribution of the overall circumferential tensile stiffnesses between the middle part and the axially outermost parts of the crown reinforcement, the middle part being stiffer than the axially outermost parts of the crown reinforcement. The reinforcing elements used in the crown or carcass layers are made of aliphatic polyamides, aromatic polyamides or are hybrid.
The technical solutions put forward in the three aforementioned documents of the prior art are still, however, insufficient in terms of reducing uneven tread wear of tires fitted to commercial airliners which have high demands placed upon them.
Document WO 2010000747 describes an aeroplane tire, the nominal pressure of which is higher than 9 bar and of which the deflection under nominal load is greater than 30%, comprising a tread having a tread surface, a crown reinforcement, comprising at least one crown layer, a carcass reinforcement comprising at least one carcass layer, the said tread surface, crown reinforcement and carcass reinforcement being respectively geometrically defined by initial meridian profiles. According to the invention, the initial meridian profile of the crown reinforcement is locally concave over a middle part of an axial width at least equal to 0.25 times the axial width of the crown reinforcement.
The technical solution described in document WO 2010000747 allows an increase in the wear life of an aeroplane tire by limiting the differential wear of the tread between a middle part and the lateral parts axially on the outside of this middle part.
While the tire lasts longer because the wear across the width of the tread is more even, its endurance performance needs to be guaranteed throughout its longer life thanks to this better wear pattern. In particular, the endurance of the crown of the tire, i.e. its ability to withstand over time the heavy mechanical demands placed on the tire, needs to be improved. Heavy mechanical demands means, for example and nonlimitingly, in the case of a commercial airliner tire, a nominal pressure in excess of 15 bar, a nominal load in excess of 20 tonnes and a maximum speed of 360 km/h.